A twisted nematic (TN) system is widely used in a liquid crystal display at present. The liquid crystal display of the TN system has a high image display contrast, but has a problem that the viewing angle dependence is large. In order to improve viewing angle characteristics of the TN type liquid crystal display, various methods have been proposed including a pixel domain division technology. However, another problem that the liquid crystal display of the TN system has a low response speed is not yet resolved. That is, a response speed of the liquid crystal display which uses nematic type liquid crystal is generally low, and a response time between gray scales becomes approximately 100 milliseconds (ms) at its maximum, so that it was impossible to realize a response time of 16.7 ms which is required for displaying a high speed moving picture. Therefore, a display system is required which is suitable for a liquid crystal display capable of displaying a moving picture and which has a wide viewing angle and a high response speed.
As generally known, an OCB type liquid crystal display has a wide viewing angle and a high response speed. A liquid crystal cell used in the OCB type liquid crystal display has bend alignment, and is also called a π-cell. It is also known that the π-cell has a high response speed.
FIG. 13 shows an example of a basic structure of an OCB type liquid crystal display. The OCB type liquid crystal display of FIG. 13 has an active matrix substrate 26 and an opposing side substrate 27 which are opposed to each other via a gap of a predetermined width such that rubbing directions of the active matrix substrate 26 and the opposing side substrate 27 become parallel to each other. Between the active matrix substrate 26 and the opposing side substrate 27, there is disposed a liquid crystal layer 25 having a bend alignment condition. The liquid crystal layer 25 put between the active matrix substrate 26 and the opposing side substrate 27 is also put between negative birefringence compensation films 28 composed by using discotic liquid crystal that is optically negative and that has a structure in which an inclination of major axis varies within the film. On both outsides of the negative birefringence compensation films 28, there are further disposed a pair of polarizer films 29. Because of the structure thereof, the bend alignment always has self compensating characteristics in the rubbing direction, and shows an optically symmetrical characteristics. In the bend alignment, variations in alignment of liquid crystal molecules becomes maximum in the optical axis, that is, within the surface which is parallel to the alignment direction of liquid crystal molecules at the interface and which is perpendicular to the substrates. Therefore, in case the bend alignment structure is sandwiched between a pair of polarizer films having mutually orthogonal polarizing directions, birefringence becomes maximum when the direction of the optical axis is disposed at an angle of 45 degrees with respect to the transmission axes of the polarizer films. When the rubbing direction is fixed to the horizontal direction, each of the transmission axes of the two polarizer films 29 is disposed at an angle of 45 degrees.
As methods of driving the OCB type liquid crystal display, there are two methods, i.e., a normally black driving method in which black is displayed on the low voltage side and a normally white driving method in which black is displayed on the high voltage side. In the normally black driving method, birefringence to be compensated is relatively large and light leakage caused by wavelength dispersion becomes large. Therefore, in the normally black driving method, it is difficult to obtain sufficiently large image contrast. Thus, such problem is solved by using the normally white driving method in which two negative Birefringence compensation films are used as shown in FIG. 13. That is, on the high voltage side, most of the liquid crystal molecules except those located near the interfaces are aligned vertically. Remained birefringence at both interfaces is compensated respectively by using two negative birefringence compensating films, and thereby wide viewing angle characteristics are obtained.
While the OCB type liquid crystal display has superior characteristics such as wide viewing angle characteristics and a high response speed, the OCB type liquid crystal display has a large problem to be solved. The bend alignment cell which is used in the OCB type liquid crystal display has splay alignment in the initial alignment condition. It is necessary to change the alignment condition of the liquid crystal molecules of all the pixels from the splay alignment to the bend alignment when a power source is turned on. Also, during a display operation, it is necessary to continuously apply a voltage equal to or larger than the critical voltage Vc in which bend alignment becomes more stable than splay alignment.
The critical voltage Vc is obtained by calculating variations of Gibbs energy in the splay alignment and in the bend alignment with respect to applied voltages, from various parameters such as matter physics parameters of liquid crystal materials, a gap of liquid crystal display panel, a pretilt angle and the like, and by comparing Gibbs energy between both alignments. Liquid crystal molecules having smaller Gibbs energy are more stable than those having larger Gibbs energy. Therefore, it is preferable that relationships between Gibbs energy and applied voltages are plotted in a graph in which an ordinate designates Gibbs energy and an abscissa designates applied voltages. From such graph, it is possible to read a voltage value at the intersection between Gibbs energy curves to obtain the critical voltage Vc.
FIG. 14 shows an example of calculation of Gibbs energy. In FIG. 14, an abscissa designates applied voltages and an ordinate designates Gibbs energy. Also, in FIG. 14, a relationship in bend alignment is shown by a solid line curve, and a relationship in splay alignment is shown by a dotted line curve. An applied voltage at which Gibbs energy in splay alignment and Gibbs energy in bend alignment become equal to each other is determined to be the critical voltage Vc. Theoretically, when a voltage larger than the critical voltage Vc is applied to a liquid crystal display panel, the bend alignment becomes more stable than the splay alignment. However, in order to cause transition from splay alignment to bend alignment, it is necessary to apply a voltage which is considerably higher than the critical voltage Vc. When a voltage of 20 volts or so is applied to a liquid crystal display panel, transition from the splay alignment to bend alignment can be done in a short time of several seconds or shorter. However, when based on the premise that a liquid crystal display is an active matrix type liquid crystal display, it is only possible to apply approximately 5 volts at the maximum because of the limitation by the withstanding voltage of a thin film transistor. It has been found by experiment that, when 5 volts are applied, transition from splay alignment to bend alignment does not occur or hardly occur. In order to cause initial transition to occur even at 5 volts or so, various methods are proposed in which transition nuclei as sources of the transition are produced to prompt the initial transition.
Technologies for providing nucleus generating means in pixel areas are as follows.
In Japanese patent laid-open publication No. 09-218411, micro-pearls are disposed between substrates of a liquid crystal display panel and used as nucleus generating means as well as spacers for maintaining the gap between the substrates. The micro-pearls have a characteristic that, at the surface of each micro-pearl, liquid crystal molecules are aligned parallel to the surface. Therefore, it is possible to stably maintain bend alignment condition while using a conventional fabrication process. However, in this method, it is necessary to disperse many micro-pearls uniformly in the liquid crystal display panel to maintain bend alignment stably. Also, in the periphery of each of the micro-pearls which function as nucleus generating means, alignment of liquid crystal molecules is distorted and, therefore, light leakage occurs when a black image is displayed.
In Japanese patent laid-open publication No. 10-142638, micro-pearls are used each of which has a diameter smaller than a gap between substrates of a liquid crystal display panel and which have a characteristic that, at the surface of each micro-pearl, liquid crystal molecules are aligned perpendicular to the surface. Thereby, liquid crystal molecules above each micro-pearl are aligned perpendicularly to the substrate and quasi-hybrid alignment is produced, so that transition from splay alignment to bend alignment is prompted to occur. However, in this liquid crystal display panel, liquid crystal molecules are aligned perpendicularly to the surface of each of the micro-pearls as nucleus generating means. Therefore, liquid crystal molecules on the sides of each micro-pearl are aligned parallel to the substrates, and light leakage becomes large. Also, in addition to dispersing gap members between the substrates, it is necessary to disperse the micro-pearls which become nucleus generating means and each of which has a smaller diameter than the gap between the substrates. Therefore, it is difficult to stably fix the micro-pearls each having a smaller diameter than the gap between the substrates.
In Japanese patent laid-open publication No. 10-020284, a convex portion having a taper shape is formed on each pixel electrode, and thereby forming an area where an electric field strength is partially high to provide a nucleus generating means. The convex portion is made of a material which has a dielectric constant larger than that of a liquid crystal material or is made of a conductive material. In this publication, a method is also described in which there is provided an area of high pretilt angle in each pixel area and liquid crystal molecules are partially pretilted at relatively high angles, thereby a nucleus generating means is provided. However, these methods have a problem that alignment of liquid crystal molecules is distorted in the periphery of the nucleus generating means and therefore light leakage occurs when a black image is displayed. In addition to such problem, these methods also have problems that process steps in manufacturing the liquid crystal display increase because of the formation of the nucleus generating means, that control of the taper shape is difficult, and the like.
As methods of providing nucleus generating means outside pixel areas, there are known the following methods.
In Japanese patent laid-open publication No. 2000-330141, a hybrid type alignment film comprising a horizontal alignment component and a vertical alignment component is used to obtain a high pretilt angle such that liquid crystal molecules have bend alignment when no voltage is applied to a liquid crystal display panel. Then, ultraviolet rays are applied only to a display area to obtain a low pretilt angle such that a splay alignment is obtained when no voltage is applied to the liquid crystal display panel, thereby forming the nucleus generating means in an area other than pixel areas. However, it was difficult to control and maintain a high pretilt angle uniformly and stably by using the hybrid type alignment film.
In Japanese patent laid-open publication No. 2000-321588, a liquid crystal display is described in which a space between pixel electrodes is narrowed, and a high voltage is applied to a common electrode. Thereby, a strong electric field is produced not only between the common electrode and the pixel electrodes, but also between the common electrode and scanning signal and image signal electrodes located between the pixel electrodes, so that transition from splay alignment to bend alignment can be surely done throughout the whole display area. This method is effective as a means for initial transition from splay alignment to bend alignment. However, since the high voltage is applied to the common electrode, it is impossible to stably maintain the bend alignment during a display operation.
In Japanese patent publication No. 3074640, a power-on reset signal is supplied to scanning signal electrodes from a driver system side, and thereby a strong electric field is produced between each of the scanning signal electrodes and a common electrode. At the same time, a voltage higher than the critical voltage Vc is applied between each pixel electrode and the common electrode, thereby causing transition from splay alignment to bend alignment in a short time. Also, during a display operation, similar operation is performed at predetermined time intervals to maintain bend alignment. Therefore, in this method, an initial transition can be effectively performed and, also, reset operation is performed at predetermined time intervals during a display operation to stably maintain bend alignment. However, in this method, it is necessary to write a black level image into a liquid crystal display panel by interrupting a display operation. Therefore, there is a problem that a transmittance of the liquid crystal display panel is substantially deteriorated.